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ty for the response, i think it is clear. Regarding DonaldŽs response, i wanted to ask a new one considering different conditions and weights: A1: 6 trials A2: 6 trials A3: 18 trials In session 2, the design is the same (same weights for conditions). If i want to define the contrast A1 - A3, is it correct to define it as [1 0 0 -1 1 0 0 -1]. Is the design correct? If something is wrong, i can detail it better. Thank you. Martin 2012/5/4 MCLAREN, Donald <[log in to unmask]> > The other thing to consider it to weight the contrast by the number of > trials in each run: > > A1 - 20 trials > B1 - 15 trials > A2 - 30 trials > B2 - 5 trials > > [.4 -.75 0 .6 -.25 0] > > Best Regards, Donald McLaren > ================= > D.G. McLaren, Ph.D. > Postdoctoral Research Fellow, GRECC, Bedford VA > Research Fellow, Department of Neurology, Massachusetts General Hospital > and > Harvard Medical School > Website: http://www.martinos.org/~mclaren > Office: (773) 406-2464 > ===================== > This e-mail contains CONFIDENTIAL INFORMATION which may contain PROTECTED > HEALTHCARE INFORMATION and may also be LEGALLY PRIVILEGED and which is > intended only for the use of the individual or entity named above. If the > reader of the e-mail is not the intended recipient or the employee or > agent > responsible for delivering it to the intended recipient, you are hereby > notified that you are in possession of confidential and privileged > information. Any unauthorized use, disclosure, copying or the taking of > any > action in reliance on the contents of this information is strictly > prohibited and may be unlawful. If you have received this e-mail > unintentionally, please immediately notify the sender via telephone at > (773) > 406-2464 or email. > > > > > On Fri, May 4, 2012 at 9:33 AM, Jonathan Peelle <[log in to unmask]> wrote: > >> Dear Julie, >> >> > I am currently processing fMRI data that includes 2 sessions. >> > >> > Can anyone provide more info or a reference regarding Martin's >> statement to >> > "Be aware when choosing the constrast because youŽll have both sessions >> in >> > your design matrix."? >> > >> > For example, if I have 2 sessions and 3 conditions per session: >> > 1: A B C >> > 2: A B C >> > >> > If I wanted to define the contrast A -B, would the contrast weights be: >> > >> > 1 -1 0 1 -1 0 >> > >> > or >> > >> > 0.5 -0.5 0 0.5 -0.5 0 >> >> These two contrasts will give you equivalent t statistics, but the >> contrast estimates will be different (though the relative size is the >> same), because the error scales with the contrast weights. The second >> one - where the positive and negative contrast weights sum to 1 - is >> technically the average of the conditions. But because the statistics >> come out the same, I don't think people usually get too bothered about >> it. >> >> Hope this helps! >> >> Best regards, >> >> Jonathan >> >> -- >> Dr. Jonathan Peelle >> Center for Cognitive Neuroscience and >> Department of Neurology >> University of Pennsylvania >> 3 West Gates >> 3400 Spruce Street >> Philadelphia, PA 19104 >> USA >> http://jonathanpeelle.net/ >> > > <http://www.unav.es/cima>